Multiple species of developmentally and tissue specific U1 RNA have been identified in mouse, frog, chicken and the silkmoth, Bombyx mori. The developmental and tissue specific appearance of these U1 isoforms has led some to suggest a role in differential splicing of hnRNA. The experiments outlined in this proposal are designed to elucidate gene expression control mechanisms at the RNA-splicing level using developmentally staged B. mori silk glands, follicle cells and an ovarian-derived permanent cell line. We have determined that the number and concentration of U1 isoforms is higher in a dedifferentiated cell line and developing posterior silk glands than it is in fully developed/specialized posterior silk glands. In the first series of experiments, we will determine the relative abundance of the U1 isoforms in the different cell types and developmental stages. Then, we will ascertain their sequence and secondary structure differences. Individual isoforms will be used in reconstitution experiments in which their differential assembly into snRNP/hnRNP particles, their association with intron-containing fibroin and chorion RNAs and their splicing mutations in humans, differential splicing as a result of U1 isoforms efficiency and/or specificity may prove to be the basis of some human maladies. B. mori with a well studied developmental biology and highly specialized cell types provides a unique opportunity to examine the function of these U1 isoforms.